Line of Sight, LOS, Multiple-Input Multiple-Output, MIMO, is a technology with the potential to drastically increase throughput in point-to-point radio communication links, e.g., microwave links used in a backhaul network. A communications system with LOS MIMO technology exploits multiple transmit and multiple receive antennas in order to increase data throughput over a radio “hop”. By using more than one transmit and more than one receive antenna, MIMO technology enables the simultaneous transmission of several parallel data streams. This is in contrast to a conventional radio hop with a single transmit and a single receive antenna, where only one data stream can be transmitted over the hop at any given time and direction.
In Line-of-Sight MIMO systems, there is comprised a number of transceivers. The positions of the transceivers are fixed, and the channel between them is usually clear of obstacles, which means that in a LOS MIMO system, multipath propagation cannot be relied upon to generate the conditions necessary for the MIMO system to function in the desired manner, since very little or no multipath propagation occur. In order to verify a MIMO system, i.e. to verify that the MIMO system fulfills the conditions necessary for MIMO communication, the relative phase difference and in embodiments also gain the ratio of the propagation paths from at least one transmit antenna to the receive antennas can be examined to see that they are within acceptable ranges, although ideally the relative phase difference should be ninety degrees and the gain ratio should be zero dB.
An alternative to MIMO systems is SIMO systems, Single Input Multiple Output systems, i.e. a system which only utilizes one transmit antenna and a plurality of receive antennas. The conditions which are necessary for MIMO communication are also necessary for SIMO communication. For this reason, we will now also use the term MIMO/SIMO, indicating the alternative MIMO or SIMO.
In many LOS MIMO/SIMO systems, each antenna is connected to its own receive unit, which has its own oscillator used to down-convert the received signal to a baseband or an intermediate frequency. Thus, due to the use of “individual” oscillators, there is introduced differential phase noise in the received signal at the different antennas. Due to this differential phase noise, it is difficult to measure the phase difference of received signals at the different antennas in order to determine whether the relative phase differences between propagation paths satisfy the requirements of a LOS MIMO/SIMO radio “hop”, since received phases have been corrupted by receive oscillators.
In addition to the phase differences between the signals received by the different receive antennas, the gain ration between the signals received by the different receive antennas is also of interest to measure in order to examine if the requirements of a LOS MIMO/SIMO system are fulfilled.
Thus, there exist specific requirements on a LOS system in order to enable MIMO/SIMO communication, and it can be difficult to verify that these requirements are fulfilled.